The present invention generally relates to satellite communication systems. More particularly, the present invention is directed to a satellite communication system capable of operating in a xe2x80x9cmobile-to-mobilexe2x80x9d mode in which mobile units communicate substantially directly via a satellite.
At the present time, there are many proposals for mobile satellite systems, in which orbiting satellites would support communication (including voice, data, and fax) with dual-mode (cellular/satellite) mobile or handheld phones. FIG. 1 illustrates a block diagram of a satellite communications system. An orbiting satellite 110 is in communication with at least one ground station or outstations called the HUB 100 as well as with a number of portable mobile stations or phones 120. The phones are each serviced by an appropriate antenna beam from a multiple spot-beam antenna on the satellite providing high gain in the direction of each phone. The HUB communicates with the satellite using, for example, C-band or K-band frequencies, while the satellite communicates with the phones using, for example, L-band or S-band frequencies. Such a satellite system can operate in multiple modes. In a first mode, communication links can be established between communication devices (e.g., standard telephone or mobile telephone) which, as a result of the respective locations of the communication devices, incorporate Public Switched Telephone Network (PSTN) equipment. In a second mode, a so-called xe2x80x9cmobile-to-mobilexe2x80x9d mode, communication links are established substantially directly between mobile units via the satellite. The xe2x80x9cmobile-to-mobilexe2x80x9d mode substantially reduces the signal delay by allowing a first mobile to communicate with a second mobile directly via the satellite transponder without requiring the signal of the transmitting mobile to go to the HUB then back up to the satellite, and finally to the receiving mobile. Mobile-to-mobile communication is discussed in detail in the copending, commonly assigned application entitled xe2x80x9cMethod and Apparatus for Enabling Mobile-to Mobile Calls in a Communication Systemxe2x80x9d, filed on Jul. 30, 1996 and having Ser. No. 08/681,916, the entirety of which is incorporated by reference.
A satellite communication system can have a symmetrical or non-symmetrical air interface. In a symmetrical air interface, both the downlink and uplink carrier waveforms occupy the same channel bandwidth. In a non-symmetrical air interface, the downlink carrier waveform occupies a first channel bandwidth (e.g., 200 kHz) and the uplink carrier waveform occupies a second channel bandwidth (e.g., 50 kHz). In a typical non-symmetrical satellite system operating in a mobile-to-mobile mode, a converter located in the satellite or in the ground station converts signals between the first and second carrier bandwidths. Mobile units in such a system transmit communication signals at a first carrier bandwidth (e.g., 50 kHz) and receive communication signals at a second carrier bandwidth (e.g., 200 kHz). A converter adds power consumption, weight, cost and complexity to the satellite. Accordingly, it would be desirable to eliminate the converter, such that mobile units operating in a mobile satellite system would be able to communicate in a xe2x80x9cmobile-to-mobilexe2x80x9d mode without requiring a carrier bandwidth conversion at the satellite.
The present invention overcomes the above-described problems, and provides additional advantages, by providing for a method, system and mobile unit which enables a non-symmetrical mobile satellite system to operate in a xe2x80x9cmobile-to-mobilexe2x80x9d mode without requiring carrier bandwidth conversion at the satellite. More particularly, according to a preferred embodiment of the invention, an additional mode capability is added to the receiver of each mobile unit, such that, when in the mobile-to-mobile mode, the receiver can receive the first carrier bandwidth (50 kHz) without requiring conversion to the normal second carrier bandwidth (200 kHz). The mobile unit is provided with two receiver paths and a switch. When in a first mode (PSTN-to-mobile), the switch is controlled to select the first receiver path so as to receive signals at the normal second carrier bandwidth. When in the second mode (mobile-to-mobile), the switch is controlled to select the second receiver path so that the mobile unit can receive unconverted signals at the first carrier bandwidth.
According to the present invention, a non-symmetrical air interface in a satellite communication system can be supported without requiring bandwidth conversion to be performed at the satellite, thus reducing payload hardware and software.